This invention relates to an underground excavator having a chamber defined in the forward section and a rotary cutter mounted to the forward end of a rotary shaft positioned to penetrate through the chamber, such that excavated materials such as soil, sand, gravel, clay and their mixture are excavated by the cutter penetrating the chamber.
The underground excavator of the kind referred to is to be placed in a vertical shaft bored in the ground and driven horizontally from the shaft into the ground to excavate the same. The excavator is effective for the installation of pipes, for example, with Hume concrete pipes or the like, for boring a tunnel through the ground composed of a high gravel content.
One method for installing pipes underground includes boring a vertical shaft into the ground to a desired depth for embedding the pipes. A leader pipe is driven through the vertical shaft horizontally into the ground. Other pipes are then pushed through the leader pipe. However, driving the leader pipe from the rearward end becomes impossible when the resisting force of the ground is large; or, if the gravel content is high, the gravel accumulating at forward end of the leader pipe hinders the leader pipe and following pipes from advancing or at least causes them to deviate from the intended route.
Japanese Patent Publication No. 57-39359 of M. Mituwa, suggests an underground excavator which uses an excavating cutter instead of the foregoing leader pipe to excavate the ground and to crush the gravel. More specifically, this excavator includes a chamber defined in the forward section for receiving therein the materials excavated. A rotary shaft and an excavating cutter, having tunnel-face ground cutting and gravel crushing bits at the forward end, are positioned to penetrate through this chamber. This underground excavator is more effective through ground having a large resisting force or high in gravel content than the conventional apparatus employing only the leader pipe, because removing hindering ground matter improves excavating ability and velocity. However, there still have been problems in that concurrently providing the gravel crushing bits and the ground cutting bits is expensive. Also, when the ground is a mixture of relatively soft ground stuffs with a high gravel content, excavation is almost impossible. This is because the low holding force of the ground for the respective gravel pieces yields only a small stress in the gravel pieces even when hit by the gravel crushing bits so that the gravel stays in the advancing route of the excavator.
Japanese Patent Publication No. 61-30118 of M. Araki suggests another underground excavator having a crushing chamber defined in the forward section of the excavator. A plurality of roller bits of generally conical-shape for crushing larger gravel, a rotary shaft to penetrate through the chamber, a spoke-shaped excavating cutter mounted to the forward end of the shaft; and excavating bits are disposed therein. This excavator crushes the gravel between the larger-gravel crushing roller bits formed on the rearward face of the cutter and the inner peripheral crushing wall of the crushing chamber to prevent the larger gravel, in particular, from accumulating in the advancing route of the excavator. This excavating function is an improvement over the subject excavator of the foregoing Publication No. 57-39359. However, using the larger gravel crushing arrangement and coupling the rotary shaft to a high output driving power source in order to crush ground of a variety of hardnesses and sizes complicates the apparatus and increases manufacturing costs. Further, the larger-gravel crushing bits are rapidly worn out due to repeated contact with the gravel for its crushing, and there has been a problem in the durability.